Fuel economy is a major consideration in engine design. One fuel savings technique that is frequently used in automotive engines is referred to as deceleration fuel cut-off (DFCO—sometimes referred to as deceleration fuel shut-off, DFSO). This mode of operation is typically used during deceleration of an engine/vehicle, when no torque request is present (e.g., when the accelerator pedal is not depressed). During DFCO, fuel is not injected into the cylinders thereby providing a corresponding improvement in fuel economy.
Although deceleration fuel cut-off improves fuel efficiency, it has several limitations. Most notably, although fuel is not injected into the cylinders, the intake and exhaust valves still operate thereby pumping air through the cylinders. Pumping air through the cylinders has several potential drawbacks. For example, most automotive engines have emissions control systems (e.g. catalytic converters) that are not well suited for handling large volumes of uncombusted air. Thus, operation in a deceleration fuel cut-off mode for extended periods of time can result in unacceptable emissions levels. Therefore, operation in a DFCO mode is typically not permitted for extended periods of time and often involves undesirable emissions characteristics. Additionally, work is required to pump air through the cylinders, which limits the fuel savings.
In principle, the fuel savings associated with DFCO can be further improved by deactivating the cylinders such that air is not pumped through the cylinders when fuel is not delivered rather than simply cutting off the fuel supply. This cylinder deactivation approach may be referred to as deceleration cylinder cutoff (DCCO) rather than DFCO. Deceleration cylinder cutoff offers both improved fuel economy and improved emissions characteristics. The fuel economy improvement is provided in part by the reduction of losses due to pumping air through the cylinders. Fuel economy may be further improved by operating in DCCO mode for longer time periods than DFCO mode, since oxygen saturation of an exhaust system catalyst is less of an issue. The emissions improvement is due to the fact that large volumes of air are not pumped through the cylinders into the exhaust system during DCCO.
Although deceleration cylinder cutoff offers the potential of significant improvements in fuel economy and emissions characteristics, it involves a number of challenges that have hindered its commercial adoption. Indeed, the applicants are not aware of DCCO being used in commercial vehicle applications. Therefore, improved engine control strategies that facilitate the use of deceleration cylinder cutoff would be desirable. The present application describes techniques and control strategies that facilitate the use of deceleration cylinder cutoff.